1. Field of the Invention
The present invention relates to an optical transmission device transmitting and receiving a radio band signal in a burst manner by an optical fiber, and a radio communication system using the above-described optical transmission device.
2. Description of the Background Art
Conventionally, a radio optical fiber (Radio On Fiber: hereinafter, referred to as ROF) technique utilizing a wide bandwidth advantage of an optical fiber for transmitting a radio band signal by the optical fiber is known. Japanese Patent Laid-Open Publication No. H05-136724, for example, discloses the ROF technique. The above-described ROF technique is used, for example, in communications between a center station and an antenna side base station in a mobile communication system. FIG. 27 is a block diagram showing the structure of the mobile communication system using the ROF technique. The system shown in FIG. 27 includes a center station 1, antenna side base stations 2, optical fibers 3, and mobile phones 4, and allows the mobile phone 4 to perform bi-directional communications with another mobile phone 4 in the system.
Radio communications are performed between the antenna side base station 2 and the mobile phone 4. The optical fiber 3 interconnects the center station 1 and the antenna side base station 2, and optical communications are performed therebetween. The center station 1 is connected to other center stations (not shown). In order to allow the mobile phone 4 to perform bi-directional communications with another mobile phone 4 in the system, the center station 1 receives a signal transmitted from the mobile phone 4, and transmits the received signal to the antenna side base station 2 or other center stations.
When transmitting a signal to the mobile phone 4, the center station 1 converts a transmission signal into a radio signal. Then, the center station 1 converts the obtained radio signal into an optical signal, and sends the obtained optical signal to the optical fiber 3. The antenna side base station 2 converts the optical signal transmitted over the optical fiber 3 into an electric signal, and transmits an electric wave based on the obtained electric signal. The electric wave transmitted from the antenna side base station 2 is received by the mobile phone 4. On the other hand, an electric wave transmitted from the mobile phone 4 is received by the antenna side base station 2. The antenna side base station 2 obtains a radio band reception signal based on the received electric wave, converts the obtained reception signal into an optical signal, and sends the obtained optical signal to the optical fiber 3. The center station 1 reconstructs the radio band reception signal by converting the optical signal transmitted over the optical fiber 3 into an electric signal, and obtains a reception signal by demodulating the reconstructed radio band reception signal.
FIG. 28 is a block diagram showing the structure of a conventional optical transmission device and a conventional radio communication system. Note that, in the radio communication system, FIG. 28 shows only a downlink system from a center station 100 to an antenna side base station 200. In FIG. 28, a radio transmission signal 182 is a transmission signal to be transmitted to the mobile phone 4, the transmission signal being converted into a radio signal. A transmission timing signal 181 is a signal indicating whether the radio communication system is in a transmission state or in a non-transmission state, that is, indicating whether or not the radio transmission signal 182 is generated. As shown in FIG. 29, the transmission timing signal 181 is frequency multiplexed with the radio transmission signal 182, and transmitted from the center station 100 to the antenna side base station 200.
A modulation section 101 modulates the transmission timing signal 181 using these methods such as ASK (Amplitude Shift Keying) or PSK (Phase Shift Keying). A multiplexing section 102 frequency multiplexes the signal output from the modulation section 101 with the radio transmission signal 182. A light emitting section 104 obtains a supply of a fixed bias current from a bias circuit 103, and sends an optical signal, whose intensity is modulated based on the signal output from the multiplexing section 102, to a downlink optical fiber 3a. 
A light receiving section 201 converts the optical signal, which is output from the light emitting section 104 and transmitted over the downlink optical fiber 3a, into an electric signal. A demultiplexing section 202 including a low pass filter (LPF) 251 and a band pass filter (BPF) 252 demultiplexes the electric signal output from the light receiving section 201 so as to obtain the two original signals, that is, the signal output from the modulation section 101 and the radio transmission signal 182, which have been multiplexed by the multiplexing section 102. A high frequency amplification section 204 amplifies one signal demultiplexed by the demultiplexing section 202 by a fixed amplification factor, and supplies the amplified signal to an antenna switch 205. A demodulation section 203 demodulates the other signal demultiplexed by the demultiplexing section 202, and outputs a transmission timing signal 281 varying in a manner similar to the transmission timing signal 181.
The antenna switch 205 changes the function of an antenna 206 in accordance with the transmission timing signal 281. In accordance with the transmission timing signal 281, the antenna 206 either transmits an electric wave based on the signal amplified by the high frequency amplification section 204, or receives an electric wave. When the antenna 206 receives an electric wave, the antenna switch 205 outputs the radio band reception signal received by the antenna 206 to a terminal 207. An uplink system (not shown) from the antenna side base station 200 to the center station 100 is connected to the terminal 207. The signal output from the terminal 207 is transmitted from the antenna side base station 200 to the center station 100 by the above-described uplink system.
In the case where communications between the center station and the antenna side base station are performed using the ROF technique as described above, a radio signal modulation/demodulation function is provided to the center station, not to the antenna side base station. Thus, the use of the ROF technique allows small and low cost antenna side base stations to be realized.
In the radio communication system typified by the mobile communication system, TDMA (Time Division Multiple Access), for example, maybe used in order to accommodate a plurality of terminals in a single network. Also, TDD (Time Division Duplex), for example, maybe used in order to perform multiplexed transmission of uplink and downlink signals using a single transmission path. In the radio communication system using TDMA or TDD, data is transmitted based on time division transmission technique, whereby a radio signal is transmitted in a burst manner.
Also, a permissible deviation is defined in the antenna side base station with respect to power of an electric wave emitted from the antenna. Therefore, the antenna side base station has to be provided with an automatic power control circuit (hereinafter, referred to as APC circuit) stabilizing the power of the electric wave emitted from the antenna. Thus, the antenna side base station included in the mobile communication system using TDMA or TDD has to be provided with the APC circuit for a radio signal transmitted in a burst manner (hereinafter, referred to as a burst radio signal).
As the APC circuit for the burst radio signal, an APC circuit (FIG. 30) disclosed in Japanese Patent Laid-Open Publication No. 2002-16506 is known. In FIG. 30, a variable gain circuit 301 and a power amplification circuit 302 amplify a modulated transmission signal. A switch circuit 303 is controlled based on a transmission control signal so as to be switched between an ON state and an OFF state, and outputs the amplified transmission signal intermittently. The transmission signal output from the switch circuit 303 is transmitted from an antenna 305 as an electric wave. A directional coupler 304 branches the transmission signal output from the switch circuit 303. A detection circuit 306 finds a power level of the transmission signal branched by the directional coupler 304. In a transmission state, a detection hold circuit 307 outputs a detected output from the detection circuit 306. In a non-transmission state, on the other hand, the detection hold circuit 307 holds the detected output in the previous transmission state and outputs the held output. A gain control circuit 308 compares a level of the signal output from the detection hold circuit 307 with a pre-set reference level, and controls a gain of the variable gain circuit 301 so as to reduce the difference between the above-described two levels.
In the above-described APC circuit, the detection hold circuit 307 outputs, to the gain control circuit 308, the signal whose level is substantially equal to the above-described reference level irrespective of whether it is in the transmission state or in the non-transmission state. Thus, the gain control circuit 308 outputs a gain control signal whose level is substantially constant. As a result, when the non-transmission state is switched into the transmission state, it is possible to quickly stabilize the level of the amplified transmission signal (the level corresponding to power of the electric wave emitted from the antenna 305).
However, the above-described conventional optical transmission device and the conventional radio communication system have the following problems. First, in the conventional optical transmission device, noise such as relative intensity noise (RIN) caused in the light emitting section or thermal noise caused in the light receiving section is amplified in the high frequency amplification section of the antenna side base station. As a result, in the non-transmission state of the radio signal, the antenna produces extraneous emissions, or noise in the downlink system has an adverse effect on the uplink system in the antenna side base station, thereby degrading the sensitivity of the uplink system in the non-transmission state of the radio signal. Also, in the conventional optical transmission device, the transmission timing signal is frequency multiplexed with the radio transmission signal and transmitted, which results in the high cost of the device due to the complicated circuits of the center station and the antenna side base station. Furthermore, in the optical transmission device, in general, a change in signal power with a change in optical power fluctuates in proportion to the square of the optical power. Thus, in order to accommodate a change in signal power with a change in optical power, the APC circuit has to be able to control the gain over a wide range of the optical power. As a result, there arises a problem that the APC circuit has to be provided with a high-performance variable attenuation circuit or variable amplification circuit.